The home button behavior is on purpose and has been there since the first steam controller. If steam is not open it opens steam, if it is then it opens big picture mode. I've only ever had it happen when not in a game so unless you are trying to open something like the Xbox game bar or using the button as your own shortcut I'm not sure when you would run into that problem. Sucks there isnt a setting to turn it off though I guess. Sorry bud.